78 PAT ike Ve [VoL. XII. 
stands, we could tell which was the new tail and which the old. 
This can be done in Fig. 96, for it is obvious that both halves 
of the tail lobe of embryo A, probably as far up as the last left 
thoracic appendage, are those of the original embryo. Embryo 
B, however, has carried off an entirely new tail lobe. The result 
is that the right half of embryo 4 will consist of the right half 
of the original embryo, and all its left half will be a new forma- 
tion, except the abdominal part. In embryo 4, its left half is 
that of the old embryo, except the posterior end, which is 
new, and was probably produced by the backward regeneration 
of that half of the body. Its right side is entirely new. A 
similar condition must prevail in Fig. 94, except that embryo 
B has here carried off the o/d abdominal lobe, and A the new 
one. 
These conditions are best seen in the diagrams, Figs. 8 and 
9, where the shaded portions represent the old parts, and the 
light ones the new. In Fig. 9, embryo J pushes past the left 
side of A and carries off a new heart and tail lobe, and the 
posterior portion of the abdomen on the left side. 
In the well-balanced condition seen in Fig. 97, it is unlikely 
that further changes in the relative positions of the two 
embryos would take place. But if they should be forced to 
grow past each other, a different proportional combination of 
old and new elements would be produced from that in Fig. 96, 
and the cause of this would be due, in part at least, to the 
different periods in the formation of the new halves at which 
the embryos separated from each other. 
In Fig. 95 the two embryos, arising by longitudinal fission as 
in the preceding ones, are still united tail to tail, but degenera- 
tion of the lower one has progressed so far as to reduce it to a 
slipper-shaped thickening with a depression in its centre, from 
which arises a papilla, probably representing the last trace of 
the fused sixth pair of thoracic appendages. A dark-rimmed 
depression in front of this probably represents the remnants of 
another pair. 
In Fig. 98, division has produced two nearly complete em- 
bryos, the new half of the abdomen of each embryo being 
absent. The lower embryo has already begun to degenerate, 
